FIG. 1A is a schematic diagram of the main structure of a conventional cursor control device. The cursor control device shown in FIG. 1A includes a ball housing 1, an X-direction and a Y-direction grid wheels 2, an X-direction and a Y-direction transmission rollers 3, an X-direction and a Y-direction light-shielding plates 4, a Z-direction urging device 5 and a ball 6, wherein the Z-direction urging device 5 further includes a Z-direction recess 51, a spring 52 and a roller 53, as shown in FIG. 1B. The conventional cursor control device utilizes the urging device 5 installed in the Z direction to improve the precision of the shift detection. The spring 52 urges the roller 53 against the ball 6 so that the ball 6 is further urged against the X-direction and Y-direction transmission rollers 3. Accordingly, the X-direction and Y-direction transmission rollers 3 can be in close contact with the ball 6 to synchronously rotate with the ball 6.
However, if the ball 6 is urged against the transmission rollers 3 only by the urging device 5, the close contact situation between the ball 6 and the transmission rollers 3 may be destroyed easily. For example, if the cursor control device is accelerated to have the ball 6 acceleratively move in a direction away frown the Z-direction urging device 5, the ball 6 may press the urging device 5 and become out of contact with the transmission rollers 3 because of inertia. On the other hand, a relatively great frictional force may also result in this defect. Accordingly, the rotation of the ball 6 cannot drive the transmission rollers 3 and further the grid wheels 2 so that the shift detection will be adversely influenced.
In addition, the only urging force exerted on the ball 6 is the elastic force of the spring 52. When the urging force is greater than the elastic limit of the spring 52 or the spring 52 becomes elastically fatigued due to a long duration of use, the close contact situation between the ball 6 and the transmission rollers 3 may also be changed.